This invention relates to papermaker's fabrics and more particularly to a papermaker's fabric manufactured from monofilament yarns encapsulated with a polymer coating to impart antisticking characteristics to the fabric and also to increase the stability of the fabric.
Forming fabrics for use in papermaking machines usually are in the form of a fine mesh cloth which has been woven endless or otherwise joined into an endless web.
At one time, all forming fabrics were manufactured from metal wires. These metal-wire cloths were useful in all kinds of papermaking machines and for all paper qualities. Eventually, metal-wire cloths were replaced by single-layer cloths or wires of synthetic fiber threads.
The advantage of synthetic threads beyond metal-wire threads primarily resides in their improved fatigue and wear resistance. Single layer synthetic wires or forming fabrics do, however, suffer from the disadvantage of having considerably less stability than paper forming cloths made from metal-wires of corresponding coarseness.
Furthermore, with such forming fabrics formed of synthetic polymer materials it is desirable to provide the polymer yarn surfaces with special surface properties to prevent the adhering of these suspended particles to the fabric which would thereby reduce the drainage properties of the fabric. Adhering of such particles is an acute problem in the case where the liquid suspension, or pulp used to make the paper contains suspended particles of filler clay, pitch (fatty acids) and/or polymer materials from repulped, deinked paper, in addition to suspended cellulose fibers. For example, a non-treated fabric installed on a corrugating machine could lose as much as 20% permeability during its useful life due to particle retention on the fabric.
In the past, the woven fabric was coated or treated, with a coating material such as is disclosed in U.S. Pat. No. 3,032,441 issued to Beaumont and Christie so as to improve characteristics of the fabric. Such a fabric treatment or coating with a film-forming polymer suspension or solution will, however, reduce the drainage capacity of the fabric because coating the entire fabric after it is woven reduces the size of each open area (interstices) on the fabric. Such a fabric treatment with a film-forming polymer suspension or solution will also, especially on high mesh fabrics, have a tendency to form a polymer film window over entire individual interstices, thus completely blocking the drainage through those particular interstices. Such blocked interstices not only reduce the overall drainage capacity of the fabric, but they will also produce quality defects in the paper (pinholes, light spots).
Westhead U.S. Pat. No. 4,274,448 discloses a dryer fabric in which stuffer or filling yarns are encapsulated in bulky fibrous materials such as mineral fibers, natural fibers and synthetic fibers to provide bulk and softness to the finished fabric. Such an encapsulating coating does not prevent suspended particles from adhering to the fabric. Further, Westhead does not teach pre-encapsulating all the yarns which contact the pulp during the paper formation process.
However, if a fabric is manufactured from encapsulated monofilament yarns, according to the present invention, the problem with reduced drainage capacity or blocking of entire interstices cannot occur.
Furthermore, if the encapsulating material has a melting point lower than the heat set temperature of the fabric, the encapsulating material would "weld" the encapsulated yarns in the fabric together at the yarn interlacings, thus producing a fabric with greatly improved stability and firmness.
This welding at the crossovers produced during finishing of a fabric of present invention will have greatly improved resistance to bowing and skewing during operation.